This invention relates to epitaxially grown semiconductors.
To generate a picosecond or subpicosecond electrical transient from a laser pulse requires the use of a specially processed semiconductor, most likely low-temperature grown GaAs (LT-GaAs). A photoconductive switch based on this material can respond quickly to short laser pulses and yields fast electrical transients when configured as a pulse generator. It can also be configured as a sampling gate to enable a brief (picosecond) segment of an unknown electrical waveform to be sampled and measured. By sequentially sampling all segments making up the electrical waveform, the shape of the waveform can be reconstructed and displayed, which is the essence of a sampling oscilloscope. The faster the sampling gate, the faster an electrical waveform that can be measured. Radiation damaged silicon on sapphire can also be used as the base semiconductor for a photoconductive switch, although it possesses less desirable photoconductive properties than LT-GaAs. A condition for good switching efficiency is that the laser's wavelength be strongly absorbed in the semiconductor. For GaAs, that means a wavelength of 880 nm or shorter, which corresponds to a photon energy equal to the bandgap of GaAs, which is 1.42 eV. A wavelength longer than 880 nm passes through the semiconductor without being significantly absorbed.
The requirement for strong absorption stems from the need to have all the photogenerated electron-hole pairs (carriers) reside in the high electric field region of the photoconductive gap. Carriers formed deeper than this region play no appreciable role in the photoconductive process. The electric field has a depth approximately that of the electrode spacing (0.50-2.0 μm) that forms the photoconductive switch, although the strongest field lines are those within the first micrometer of the surface.
Femtosecond (10−15 s) pulse lasers do exist that absorb well in GaAs (and silicon) and have been used for the past 25 years to generate picosecond and subpicosecond electrical pulses. Such lasers, one of the most common of which being Ti:sapphire (lasing at 800 nm), are large, water-cooled and expensive to purchase and maintain. They cannot be amplified using optical fiber amplifiers, and fiber components at this wavelength are difficult (if not impossible) to manufacture, requiring instead the use of free-space optics.
The ideal source is one that is compatible with other telecom components and can be directly pumped with a common semiconductor pump laser. It has sufficiently broad emission band to support femtosecond pulses. It also has a wavelength suitable for fiber amplification. Additionally, it is power-efficient, air-cooled, compact and Telcordia-qualified for maintenance-free, long-life operation. New telecommunication laser technologies have made possible two such lasers, classified by their operating wavelengths. They are: Er:Glass lasers operating at 1550 nm and Nd:Glass or Ytterbium, both operating at 1060 nm. These wavelengths generate near-zero photocurrent in GaAs. To take advantage of these new sources requires development of a new semiconductor tailored to these wavelengths.
To use either of these wavelengths means that the semiconductor's bandgap must be made equal to or less than the photon energy of the laser light. The approximate bandgaps for 1060 nm and 1550 nm are 1.15 eV and 0.8 eV, respectively. A commonly used semiconductor in the telecommunications industry, In0.53Ga0.47As grown on InP, does have a bandgap of 0.77 eV and strongly absorbs light out to 1650 nm. Unfortunately, this semiconductor like all reduced-bandgap semiconductors suffers a serious limitation when configured as a photoconductive switch.